Low-power Complementary Metal-Oxide Semiconductor (CMOS) reference clock oscillators may be used in miniaturized system-on-chips (SoCs) for emerging microsystems such as implantable biomedical devices and smart sensors, which require low power consumption and small system sizes. In such miniaturized SoCs, the supply voltage may be low while the level of analog and digital circuit integration may be high to meet rigorous power and area constraints. As such, noise from other blocks, especially digital blocks coupled through supply and ground lines, may become a serious threat that degrades the performance of CMOS reference clock oscillators.
Low-power CMOS reference clock signal generators may be based on relaxation oscillators which may provide frequency stability and low noise. State-of-the-art reference clock signal generators may not have immunity to noise from supply and ground lines, as the state-of-the-art reference clock signal generators may include supply-sensitive building blocks, such as reference voltage generators and single-ended comparators. One solution to overcome the noise from supply and ground lines may be to use large external decoupling capacitors. However, large external decoupling capacitors may not be suitable for applications which need to be small in size and low in cost. Another solution for making the reference clock oscillators immune to supply noise may be to regulate the supply line with a band-gap voltage regulator. However, band-gap voltage regulators tend to be high in power consumption and may also be large in size. Moreover, even with the use of band-gap voltage regulator, decoupling capacitors of several hundreds of nanofarads may still be required to remove high frequency noise, as the power supply rejection ratio of band-gap voltage regulators may start to deteriorate from around tens of kilohertz.
Therefore, there is a need for a new reference clock signal generator which can be immune to noise from supply and ground lines, while being suitable for power and area-restricted applications.